1. Field of the Invention
The disclosed methods and systems relate generally to control techniques, and more particularly to techniques for normalizing error in semiconductor manufacturing processes.
2. Description of the Prior Art
Lithography is a process used in semiconductor manufacturing to transfer a circuit pattern from a photomask or reticle to a semiconductor wafer, or more specifically, to transfer the photomask pattern to a layer of resist that has been deposited on the wafer surface, where the resist is sensitive to irradiation. Different types of lithography may be based on the wavelength of the radiation used to expose the resist. For example, photolithography, otherwise known as optical lithography, uses ultraviolet (UV) radiation and a corresponding UV-sensitive resist. Ion beam lithography uses a resist sensitive to an ion beam, electron beam lithography uses a resist film sensitive to a scanning beam of electrons to deposit energy therein, and X-ray lithography uses a resist sensitive to X-rays.
Photolithography employs a photomask that may be understood to be a quartz plate that is transparent to UV radiation and includes a master copy of an integrated circuit that is often a microscopic integrated circuit. The photomask may be used to block resist exposure to select areas using chrome opaque areas.
A stepper is a resist exposure tool used in many photolithography systems to expose part of the wafer or resist in a given exposure. Systems employing a stepper may require a “step-and-repeat” process to expose the entire wafer as desired. A scanner is another type of resist exposure tool used in photolithography systems to expose part of the wafer or resist in a given exposure. Systems employing a scanner may require a “step-and-scan” process to expose the entire wafer as desired. In the aforementioned systems, overlay may be understood as the superposition of the pattern on the mask to a reference pattern previously created on the wafer surface. Related to overlay is alignment, which may be understood to be including positioning, or aligning, the mask or reticle relative to markers or targets on the wafer, prior to the exposure. Accordingly, to achieve proper exposure, overlay and alignment, among other parameters, should be properly controlled.
The smallest transverse dimension of a developed photoresist is commonly known as the critical dimension (CD). The critical dimension CD depends on the exposure or photoresist exposure dose, which is a measure of the light absorbed by the photoresist. Accordingly, a proper exposure dose for a given pattern may include different exposure times for different substrates based on the substrate optical properties. For example, an exposure dose may be based on the photoresist layer thickness which may change during manufacture to alter the surface's optical properties, thereby influencing the amount of light coupled into the photoresist. The CD of the developed photoresist thus determines the CD of the patterned material, and changes in a substrate's optical properties may result in unacceptable variations during the manufacturing process.
As the demand for smaller yet more complicated integrated circuits (ICs) increases, there is a similar demand for increased level of integration and reductions in the CD. Because lithography may occur repeatedly throughout IC fabrication, the CDs of the lines in the different patterns which are transferred should be precisely controlled throughout the fabrication process.
What are needed are techniques for further reducing the deviation of the critical dimension (CD) from the target value (CDT) in a lithography process. Preferably, the techniques are amenable to integration with production systems.